Buckling Mechanism And Control Of Aluminum Sheet In The Fluid Cell Forming Process

ARJ21 regional jet adopt aluminum honeycomb sandwich structure to reduce the weight,this structure consists of two aluminum panel plates and a hexagonal honeycomb core,the aluminum panel plates are double curved shallow bowl-shaped parts,the thickness of which is less than 0.5mm,and produced through fluid cell forming.The bottom and flange of parts have obvious buckling and warp defects after forming,these resulting in the parts can not pass the Non-destructive Testing,and high rejection rate,which causes great economical loss.To solve this problem,this paper analyzed the buckling defects through finite element simulation and experiment in the fluid cell forming.This paper studied on the sheet metal deformation before and after forming,the main works and innovation can be summarized as follows:1)Based on the one-step forming process,this paper used two kinds of leveling process to reduce the quenching deformation: pre-stretching leveling process and roller leveler.We obtained the control value of the sheet stretch through the theoretical analysis and experimental study on the sheet pre-stretching leveling,and got the amount of pre-stretching process in the practical production.A method of sheet curvature estimation was proposed through the theoretical analysis on the sheet roller leveling.A mixed hardening model describing the sheet suffered the cyclic loading in the leveling process was proposed.The theoretical results showed the sheet material had softening process after roller leveling,and then the experimental results showed the softening effect was decreased with aging time increasing.These had important guide significance to the optimization forming time.2)This paper studied on the bulging deformation of the parts in the fluid cell forming process.We got the strain and stress fields of the formed parts through the theoretical analysis and finite element simulation in the bulging process.The effect of cavity dimensions on the radial and tangential stress in the bulging process was studied through changing the dimensions of the cavity.Based on the results of theoretical analysis in the springback process,we obtained the relationship between the cavity dimensions,aging time,the springback of the sidewall and the buckling height of the flange.The relationship between radius of the flange and the wave number of the flange buckling was obtained through the theoretical analysis of the large deflection buckling.3)The fluid cell forming process was simulated using Pam-Stamp 2G software.Thematerial model was made through caculating the material property after leveling,and a finite element model of basin-shaped fluid cell forming parts was made based on this.We got the fields of strain and stress before and after the springback through the simulating the circular part forming process.Analyzing the results of simulation,we found the bottom material had the radial extension after springback which caused the flange circumferential compression,so that the part buckling after fluid cell forming.Finally,we found the non uniform distribution of stress and large size cavity caused the complex structure part buckling after analyzing the simulation results.4)We made a experimental study on the fluid cell forming process of the simple shallow basin-shaped parts,and analysed the effects of the forming pressure,the hardness of the rubber,lubricants and other process parameters on the buckling forming of the parts.The results showed a polymer quenching medium instead of water can reduce 70% quenching deformation before the fluid cell forming;the castor oil was applied between the die and sheet as the lubricant,it increased the buckling height;the buckling height increases with the increase of aging time and decrease of the sheet thickness;the flange buckling height and the wave number are reduced with the blank dimensions increase;the forming pressure had less effect,so that the forming process consist of pressing twice and using large blank which wrapping the die to eliminate effect of the air resistance;the hard rubber is used to forming the large cavity parts,the soft rubber correspond to the small ones.According to the experimental conclusion,the part with the double curved structure and large cavity was formed through the fluid cell forming,finally we got the parts which meet the process requirements,and the yield rate of the parts increased to 90%.